error: expression is not assignable ternary operator

问题

I have the following code and MPLABX XC8 compiler gives this error:

error: expression is not assignable

U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : uart1.oerr = 0;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^

Here is the relevant code section:

typedef union {
    struct {
        bool ferr  : 1;  // FERIF Framing Error
        bool aerr  : 1;  // ABDOVF Error
        bool oerr  : 1;  // RXFOIF Error
        bool ready : 1;  // Data Ready to be read
        uint8_t reserved : 4;
    };
    uint8_t status;
}uart1_status_t;

static volatile uart1_status_t uart1;

U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : uart1.oerr = 0;

Same thing does not give error when I use

if (U1ERRIRbits.RXFOIF) 
    uart1.oerr = 1;
else 
    uart1.oerr = 0;

Do not understand why?

回答1:

U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : uart1.oerr = 0;

is interpreted as:

(U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : uart1.oerr) = 0;

Which tries to assign 0 to ...? If you want to use such construct, you need braces:

U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : (uart1.oerr = 0);

Or better:

uart1.oerr = U1ERRIRbits.RXFOIF ? 1 : 0;

or:

uart1.oerr = !!U1ERRIRbits.RXFOIF;

or:

uart1.oerr = (bool)U1ERRIRbits.RXFOIF;

or really just:

uart1.oerr = U1ERRIRbits.RXFOIF;

as typeof(uart1.oerr) == bool, the value will be implicitly converted to 1 for nonzero values or 0 for zero.

回答2:

The answer is simple that, due to C operator precedence,

U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : uart1.oerr = 0;

is compiled as

(U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : uart1.oerr) = 0;

In C, a condition doesn't provide an LValue. Hence, this is a compiler error.

One solution would be to use parentheses:

U1ERRIRbits.RXFOIF ? uart1.oerr = 1 : (uart1.oerr = 0);

Please note, that uart1.oerr = 1 doesn't need parentheses as ? and : act like parentheses (a specialty of the ternary operator).

The even simpler solutions are already mentioned in Kamils answer...

来源：https://stackoverflow.com/questions/55377646/error-expression-is-not-assignable-ternary-operator

标签

struct

union

ternary